Re: [mu TECH] Network questions for Sven Conrad

From: Sven Conrad (
Date: Wed Sep 13 2000 - 23:04:52 CEST

Hi Michele

Michele Andreoli nicely wrote:
> Hello Sven,
> I have now three computers. The "central" node is connected to "left" node
> using eth0 and to "right" node using eth1 and ppp0. Nodes runs muLinux.
> The network is ideally a slipknot, or a cowboy's lazo, if you wish,
> because central and right node have two redundant connections.
> I declared different Class C networks for any interfaces
> (eth0) (ppp0) (eth1).
> Routing and masqueranding works nice and I can ping and telnet in
> every side.

Nice playground. Can be dazzling when you mean one interface but it
takes the other. Does it work to setup it as real loop, i.e. route
eth for one dir and ppp for the reverse? UDP should work, but did

> question 1
> -----------
> Please, isolate my doubts: because central node has three different
> IP addresses, what is the ip associated with the hostname for
> central host?
> Is that currently a senseless question? I.E., the server has no ip itself?
> Domainname is senseless? (I'm ashamed of myself for such dubt; please,
> do not publicize)

Not lazy, this is mostly underestimated. A machine has no single IP by
It can have more (like yours)! So what is the self-IP-address? It simply
depends on the destination. Because there is a unique route which will
be used for a specific destination. This route correlates to one
interface and
so to one IP. This is allso the closest IP address of this machine to
with this destination. If all routes are setted i.e. the destination
knows all subnets
and has propper routes (i.e. gateways or masqurading) a application can
any IP. But then there is a additional step in the kernel neccessary to
the packet between these IP's.

Oh my guash, does this make sense to you or generates it more questions?

So let us look for a more advanced program and it's setup: samba.
Samba graps the IP of the first adapter as default and works on this.
What if
there are more? Then you can insert setup lines if you want samba in an
other subnet or in multiple subnets. Therefore you give it IP and
This is not done automaticaly by samba.
Domainname is an other question. Each machiene should have a Hostname
a Domanename. The domain is mostly common for the entire network. The
Hostname is the machine name. But AFAIK there is only a very loose link
between Names and IP's as long as you have no central service for this.
E.g. you can give your computers different names on every workstation.
is more a problem for humans. But the Internet is only IP related, the
are for more readability and are in no way transported.
I hope you use no nameserver for this network? That is an onther story.

> question 2
> -----------
> Using "muless -r 1 /proc/net/dev" on the central node, I can monitor
> what is happens; in some case: packets appairs to *enter* in
> an interface, but to *exit* using another interface!
> That seems to me a sort of 4-space singularity in the kernel's gravitational
> subsystem.
> Is it caused by the concurrent spurious telnet session used, or is that
> the wonderful world of masquerading/forwarding in the linux kernel?
I'am not sure, if I have understood this question.
If this take place on a gateway, than this is what I expect. There is
a interface which receives a packet, which is not addressed to the IP
of this interface. So the kernel trys to deliver it to this interface.
If a route is found, this packet is stuffed into the corresponding
interface. How masqurading interfere with this is beond my knolledge.
I don't know, if it sits before or behind the IP-interface. But I expect
it on the inner side, so you will see *enter*, then the NAT take place
and as last stage this Packet is sended to the restore IP (*exit*), even
on a single machine. AFAIK is firewalling a shell around the the kernel
routing (masquerading is one functionality of the firewall).


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